1. Field of the Invention
The present invention relates to a manhole head assembly for mounting a cover onto a manhole frame.
More specifically, the invention relates to a manhole head assembly that is vertically adjustable whenever desired to conform with a new surface level.
2. Description of Related Art
Known in the art is Canadian laid-open patent application No. 2,074,093 in the name of Gaetan BEGIN, which discloses a manhole head assembly having a plurality of stacked rubber rings to be mounted onto a manhole frame. Each ring is fastened only to the one below it, with the lowermost ring being fastened to the manhole frame. Such makes the assembly cumbersome to install. Moreover, the uppermost ring has an inner peripheral recess for supporting the cover, thereby making it necessary to use a mold of complex structure to manufacture it.
Also known in the art is U.S. Pat. No. 3,237,538 in the name of James H. McPHEETERS et al., which suggests to add a plurality of annular members to an existing manhole head in order to raise the cover to a new street level. A drawback of this invention is that it can only be installed onto an existing manhole head. Another drawback to this invention is that the installation of the annular members involves using a complicated system of dowels.
Also known in the art is U.S. Pat. No. 3,408,778 in the name of Keith K. MASON, which also discloses means for raising the manhole cover of an existing manhole head, which means have the same limitation as above, i.e. they can only be used onto an existing manhole head.
Although the art includes many other patents directed to manhole head assemblies, the three aforementioned patents are believed to be quite representative of the art in this field.